Code standards help ensure that everyone has access to the information you are providing, and make it easier for people with special needs to use the web. MIT recommends that all web developers adhere to coding standards. For more on web standards go to the W3C or Web Standards Project websites. Pages should be checked with the W3C Markup Validation Service.
HTML (HyperText Markup Language)
Document layout and hyperlink specification language. The mark-up tags tell the browser how to display the contents of a document including text, images and other supported media. While the W3C has defined the standards of HTML, browser manufacturers have taken some leeway in how their browsers display pages and have added nonstandard extensions to the language.
XHTML (Extensible HyperText Markup Language)
A reformulation of HTML to be compliant under XML (Extensible Markup Language). Using the more rigid rules of XML, XHTML follows every feature of HTML 4.01. It requires more discipline and attention to detail than an HTML document. XHTML demands careful attention to upper and lower case letters, quotation marks, closing tags, and other minutiae ignored by regular HTML. The majority of HTML is completely compatible with XHTML. One of the major differences is the nesting of elements; every tag that contains other tags or content must have a corresponding end tag present. Since empty elements (e.g. <br>, <img>) don't usually have an end tag present, they must include a space and a slash before the closing brace of the tag (e.g. <br />, <img />). XHTML also requires that tags and attributes must be lowercase, and that all attributes be quoted and have an associated value.
XML (Extensible Markup Language)
Based on the same patent technology as HTML, but designed to better handle the task of managing information. Rather than serving as a language for creating web pages, XML is a language for creating other languages. XML tags identify data so the data can become available for other tasks.
HTML was never intended to deliver high-concept graphic content and multimedia. HTML was created to allow authors to define the structure of a document for distribution on the web. CSS or style sheets, allow you to control the display of a web document without compromising its structure (e.g. fonts, colors, leading, margins, typefaces, and other aspects of style). HTML tags can be redefined and custom style classes can be created to globally control the display of your web pages. Styles can be defined in an external CSS file, in an individual document, or even a specific tag. Unfortunately not all browsers offer the same level of support for CSS so you must check your work with all target browsers.